Just how bad was our July heatwave?

As we bask this week in a cool air mass (what's an air mass ?), let's take a look back on our big July heatwave! Just how hot was it? Well.

The first graph below shows the daily maximum temperatures for July 1-31 in 2006 (red), as well as in 2004 (dark blue) - a "cold" year!!

Also shown in light blue is the climatological average - the average over the past 30 Julys

As you can see from the light blue bars, our normal daily high temperature is around 82 ° F in July. At the start of July 2006 (red bars) we were actually below normal for a few days - but the heatwave made up for that! Maximum temperatures were over 90 ° F every day for during the 10-day period July 16-25. This string of hot days broke a record set in 1939 ( more details on records can be found here ). By contrast in 2004 (dark blue bars) the highest July temperature was a mere 89 ° F.

The warm nighttime "low" temperatures during the heatwave might be more impressive - the graph below shows these.

Climatologically (light blue bars), we expect overnight lows in the range of 54-56° F in July. July 2004 looks a little warmer than this, but the 2006 heatwave period shows up clearly as much warmer than average - over 15 ° F above average at its peak!! No wonder you couldn't sleep!

So - was this a result of global warming? It certainly is interesting to note that overnight lows for both 2004 and 2006 were above 30-year averages. One prediction of global warming theory is that warmer nighttime temperatures will be more noticeable than warmer daytime temperatures. This is because at night, with no solar heating, the only thing the atmosphere can do is cool down - and the warming effects of increased amounts of greenhouse gases will mitigate this.

HOWEVER - as for 2006, let us not forget that we just had a long, cold , rainy month of March (details) - so we've had an exceptionally cold spell and an exceptionally hot spell this year - they probably cancel out to give us an average year!! On the other hand, another prediction of global warming theory is that we can expect more extremes of weather . more hot spells, cold spells . food for thought.